The present invention relates to a magnetic resonance imaging (MRI) system for imaging an object based upon a function of the spin density distribution of the specific nucleus existing in a specified cross section of the object (e.g., the density distribution of the nuclear spin), the spin lattice relaxation time (T.sub.1) and the spin relaxation time (T.sub.2) of the nucleus, thereby to form an MR image.
Various MR techniques have been proposed including a projection method described below.
In this system, a sufficiently uniform static magnetic field H.sub.O is applied to object P as shown in FIG. 1 and a gradient magnetic field G.sub.z is further applied to object P due to a pair of gradient coils C.sub.la and C.sub.lb in addition to the static magnetic field H.sub.O. The direction of the lines of magnetic force of the static magnetic field H.sub.O is parallel to the Z axis shown in the diagram. The magnetic force of the gradient magnetic field G.sub.z is parallel to the Z axis shown in the diagram and this gradient magnetic field has a linearly ascending gradient with respect to the direction of the Z axis (namely, the magnetic field intensity gradually changes with respect to the Z axis). The magnetic gradient of gradient magnetic field G.sub.z is such that, for example, the magnetic field intensity in the vicinity of substantially a central portion of the object along the direction of the Z axis is zero and the directions of lines of magnetic forces before and after the central portion are oppositely directed and at the same time the magnetic field intensity gradually increases. Thus, the composite magnetic field of the static magnetic field H.sub.O and the gradient magnetic field G.sub.z also has a magnetic gradient with respect to the Z-axis direction, so that magnetic field contour planes are formed in planes perpendicular to the Z axis. Namely, in this case, a specific magnetic field contour plane corresponds to a specific magnetic field intensity wherein a substantial central portion with respect to the Z-axis direction corresponds to the intensity of magnetic field H.sub.O.
The atomic nuclei resonate under the static magnetic field H.sub.O at an angular frequency of EQU .omega..sub.O =.gamma.H.sub.O ( 1)
In expression (1), .gamma. is a gyromagnetic ratio which is peculiar to the atomic nucleus and is determined depending on the kind of atomic nucleus.
In addition to static magnetic field H.sub.O and gradient magnetic field G.sub.z, an additional, excitation magnetic field H.sub.1 corresponding to the angular frequency .omega..sub.O functions to resonate only the specific atomic nucleus, and is applied to object P as a pulse through a pair of transmitter coils C.sub.2a and C.sub.2b provided in a probe head. This pulsed field H.sub.1 is called an "excitation pulse." The excitation field H.sub.1 substantially selectively acts on only the portion of the X-Y plane shown in the diagram (which is selectively determined with respect to the Z-axis direction) due to the gradient magnetic field G.sub.z. Thus, the MR phenomenon occurs in only the specific slice portion S (although it is a plane portion, it actually has a certain thickness), from which two-dimensional or three-dimensional images are obtained.
Due to the occurrence of the MR phenomenon, a free induction decay (FID) signal is detected through a pair of receiver coils C.sub.3a and C.sub.3b provided in the probe head. This FID signal is subjected to a Fourier transformation, so that a single spectrum representative of a one-dimensional projection of the nuclear spin density integrated over planes perpendicular to the direction of the gradient is derived with respect to the rotating frequency of the specific atomic nuclear spin. To obtain two-dimensional or three-dimensional images, the projection data of slice portion S regarding multi directions of the magnetic field gradient in the X-Y plane is necessary. Therefore, after the MR phenomenon is caused by exciting slice portion S, as shown in FIG. 2, a gradient magnetic field G.sub.xy having a magnetic gradient which is linear in the direction of the X' axis (coordinate axis which is rotated by an angle of .theta. from the X axis) is allowed to act on magnetic field Hn (by a coil or the like--not shown). Thus, magnetic field contour lines E.sub.l to E.sub.n in slice portion S (X-Y plane) of object P become straight lines which perpendicularly cross the X' axis. The rotating frequencies of the specific atomic nuclear spins projected on magnetic field contour lines E.sub.l to E.sub.n are expressed by expression (1) mentioned above. Signals D.sub.l to D.sub.n corresponding to the FID signals are caused by the magnetic fields on magnetic field contour lines E.sub.l to E.sub.n, respectively. Amplitudes of these signals D.sub.l to D.sub.n are proportional to densities of the specific atomic nuclear spins on magnetic field contour lines E.sub.l to E.sub.n which pierce slice portion S, respectively. However, the FID signal which is actually observed is a composite FID signal (labeled FID in FIG. 2) made up of the sum of all of D.sub.I to D.sub.n. This composite FID signal FID is subjected to the Fourier transformation, so that projection data (one-dimensional image) PD of slice portion S onto the X' axis is obtained. The X' axis is rotated in the X-Y plane. This may be achieved by providing a gradient magnetic field G.sub.x for providing a magnetic gradient with regard to the X direction by a pair of gradient coils, and at the same time by providing a gradient magnetic field G.sub.y for providing a magnetic gradient with respect to the Y direction by another pair of gradient coils. These gradient magnetic fields G.sub.x and G.sub.y are combined to form a gradient magnetic field G.sub.xy, and by changing the ratio of the gradient magnetic fields G.sub.x and G.sub.y, it is possible to rotate or change the direction of the gradient magnetic field G.sub.xy. Due to this, the projection data regarding each direction in the X-Y plane can be derived in a manner similar to that described above. The MR image can be synthesized by an image reconstruction process using the projection data.
Although there is a case where the FID signal due to the magnetic resonance is observed by directly detecting the FID signal itself, there is also a case where the magnetic resonance is excited so as to generate a spin echo signal relative to the FID signal and this spin echo signal is detected, thereby enabling observation of the FID signal.
A 90.degree. pulse and/or a 180.degree. pulse is used as an exciting pulse in order to obtain the FID signal or spin echo signal. The 90.degree. pulse is the exciting pulse for causing such a magnetic resonance as to rotate the magnetic moment of the spin system by an angle of 90.degree. from the direction parallel to the magnetic field to the direction which is normal thereto due to the resonance. Similarly, the 180.degree. pulse is the exciting pulse for causing such a magnetic resonance as to rotate the magnetic moment of the spin system by an angle of 180.degree. . The 90.degree. pulse is mainly used to obtain the sole FID signal. Both of the 90.degree. and 180.degree. pulses are frequently used to obtain the spin echo signal.
The conventional diagnostic MRI system encounters a problem. Even if the condition to generate the 90.degree. or 10.degree. pulse has been preset, the power condition of the pulse will inevitably change since, when the object is put into the transmitter coils, the quality factor (Q) and resonance frequency (f.sub.O) of the coils alter depending on attributes of the object, for example, the shape. Thus, the angle of inclination of the magnetic moment of the spin system deviates from the set value of 90.degree. or 180.degree. when the exciting pulse is applied. Consequently, picture quality features, e.g., contrast, S/N ratio, or the like of the MR image which are obtained from the FID signal or spin echo signal, vary depending on the attributes of the object. Moreover, conventional imaging techniques take considerable time to obtain the desired series of cross-sectional images and it would be advantageous to shorten this time as much as possible without unduly destroying image quality.